This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-372621, filed Dec. 24, 2002, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method for manufacturing a substrate for a display panel which is used, for example, in manufacture of a liquid crystal display panel and, in particular, to a method for manufacturing a display panel applied to a light transmissive substrate including an organic insulating film stacked thereon and having a plurality of openings of different sizes.
2. Description of the Related Art
A liquid crystal display device is used in an apparatus, such as a personal computer, TV, word processor and hand-held telephone. In such apparatuses, a growing demand is now being made for achieving a compact, power saving, lower cost, etc., unit further including more functions. In order to satisfy such demand, a reflective type liquid crystal display device using ambient light as a light source as well as a semi-transmissive type liquid crystal display device using both backlight and ambient light as a light source is now under development.
For example, the semi-transmissive type liquid crystal device, like the transmissive type, has a display panel having a structure with a liquid crystal layer held between an array substrate and a counter substrate. Generally, the array substrate includes a matrix-like array of pixel electrodes and a corresponding array of switching elements connected to these pixel electrodes, while, on the other hand, the counter substrate includes a single counter electrode opposite to the array of pixel electrodes. Here, each pixel electrode has, for example, a reflective electrode section and a transmissive electrode section formed as a light transmissive window surrounded with the reflective electrode section. The reflective electrode section reflects, as reflecting light, ambient light incident via a liquid crystal layer from the counter substrate side, while, on the other hand, the transmissive electrode section transmits, as transmitting light, backlight incident from the array substrate side. In such a system, the difference in height between the reflective electrode section and the transmissive electrode section is set such that the LC layer thickness above the transmissive electrode section is about two times that above the reflective electrode section. By doing so, the optical condition is optimized relative to the reflecting light and transmitting light and it is possible to reduce a loss both in reflectivity and in transmittance.
In the manufacture of the array substrate, a thin-film transistor is formed as the switching element at a transparent insulating substrate such as a glass substrate and, after this, an organic insulating film is formed to cover the switching element therewith. Further, selective patterning is done to provide a first opening for receiving the transmissive electrode and a second opening acting as a switching element""s contact hole. And the first and second openings are formed by a photolithography process of exposing the organic insulating film to light with the use of a photomask having a planar pattern shown in FIG. 22 and selectively removing the exposed-light portions. After this process, a transparent conduction film such as ITO is sputtered to cover the organic insulating film and first and second openings and patterning is effected to obtain pixel electrode shapes. By doing so, the transparent conduction film contacts the switching element in the second opening. After this, sputtering is done to cover the transparent conduction film with the reflective conduction film made of a metal. And patterning is done to obtain a reflective electrode shape with the transparent conduction film exposed in the first opening. The reflective electrode section is obtained by the reflective conduction film covering the transparent conduction film around the first opening while, on the other hand, the transmissive electrode section is obtained by the transparent conduction film exposed in the first opening. Therefore, the difference in height between the transmissive electrode section and the reflective electrode section is set generally dependent upon the thickness of the organic insulating film.
Incidentally, the second opening serving as a contact hole is smaller than the first opening for receiving the transmissive electrode section. In the light exposure of the organic insulating film using the photomask shown in FIG. 22, therefore, an inadequate light exposure is liable to occur at the light exposure section for the second opening. The amount of light applied to the organic insulating film may be increased to make a contact hole in the film without fail. In this case, too much light is applied to the exposure part for the first opening. Consequently, the first opening is adversely affected by the surface condition of the light exposure machine""s stage on which the array substrate having the switching element formed on the transparent insulating substrate and covered with the organic insulating film is placed. That is, in the light exposure machine""s stage there are many holes, such as pin insertion holes for lifting the array substrate, vacuum holes for sucking the array substrate, etc. At places other than the pin insertion holes and vacuum holes, the organic insulating film undergoes a double light exposure by the light which, after passing through the array substrate as shown in FIG. 23, is reflected on the surface of the light exposure machine""s stage. As a result, the first opening is formed to have a size of L1 shown in FIG. 24, while, on the other hand, in those areas where pin insertion holes and vacuum holes are present, the light passing through the array substrate as shown in FIG. 25 is not reflected on the surface of the light exposure machine""s stage and, therefore, the first opening is formed to have a size of L2 smaller than the size of L1. Such a size variation of the first opening causes an uneven stage mark to be appeared in a reflected-light display image, thus prominently lowering a quality of display.
This stage mark can be alleviated by suppressing an increase in amount of light transmitted through the photomask. It is, however, difficult to form contact holes with high reliability. There arises the problem that the number of defective pixels in the display screen increases due to contact failures in the contact holes. If larger contact holes are initially formed, then it is not necessary to excessively increase the amount of light from a light source. However, this also decreases the effective area of the reflective electrode section. Thus, it is not possible to maintain a high reflectivity.
The present invention has been made to overcome the above-mentioned problems, and it is accordingly an object of the present invention to provide a method for manufacturing a substrate for a display panel which can obtain a uniform and excellent display quality without increasing the number of defective pixels.
According to one aspect of the present invention, there is provided a method for manufacturing a substrate for display panel, comprising a step of forming an organic insulating film on a light transmissive substrate and a step of patterning the organic insulating film to form a first opening and a second opening smaller than the first opening as through holes in the organic insulating film, wherein the patterning step comprises a photolithography process of selectively exposing the organic insulating film to light with the use of a photomask and removing the light-exposed portions, the photomask being so set that the average light transmittance is lower at a light transmissive area for the first opening than at a light transmissive area for the second opening area.
In this manufacturing method, the organic insulating film on the light transmissive substrate is selectively exposed to light to form the first opening and the second opening smaller than the first opening. This exposure is effected with use of the photomask whose average light transmittance is so set as to be lower at the light transmissive area for the first opening than at the light transmissive area for the second opening. Thus, a light exposure amount for the first opening and that for the second opening can be respectively optimized through the utilization of their average light transmittance difference. Ever if an amount of light from the light source is increased to reliably form the second opening as a through hole in the organic insulating film, the amount of light applied to the organic insulating film is set to be weaker at the light transmissive area for the first opening than at the light transmissive area for the second opening. Therefore, even if the light reaches, through the light transmissive substrate, the surface of the light exposure machine""s stage on which the substrate is placed, there never occurs any prominent variation in size of the first opening resulting from the presence/absence of light reflection depending upon the surface condition of the light exposure machine""s stage. It is, therefore, possible to obtain a uniform and excellent display quality without increasing the number of defective pixels.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.